1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing program for adaptively performing tone conversion processing on an image signal, and particularly to an image processing apparatus and an image processing program capable of performing optimal tone conversion processing in consideration of the influence of a noise component.
2. Description of the Related Art
In current image pickup apparatuses, such as a digital still camera and a video camera, to prevent deterioration of image quality due to underflow in digital signal processing, it is common to set the tone range of a signal in an input system and a processing system (approximately 10 to 12 bits) to be greater than the tone range of a final output signal (usually 8 bits).
In such a configuration, the tone conversion processing with general emphasis processing needs to be performed to adjust the tone range of a signal to the tone range of an output system.
In the above, the tone conversion of an image signal has been performed in conventional tone conversion processing with the use of a fixed tone conversion curve for a standard scene.
Meanwhile, techniques of adaptively performing the tone conversion processing have been proposed. Such techniques include, for example, a technique of calculating the tone conversion curve for each image signal on the basis of a histogram and performing the tone conversion on the basis of the calculated tone conversion curve.
Further, Japanese Patent No. 3424060 describes a technique of dividing an image signal into a predetermined number of regions, calculating the tone conversion curve for each of the divided regions on the basis of a histogram, performing the tone conversion processing on the basis of the calculated tone conversion curve, and thereafter combining the respective regions subjected to the conversion.
Further, Japanese Patent No. 3465226 describes a technique of dividing an image signal into a plurality of regions on the basis of texture information, calculating the tone conversion curve for each of the divided regions on the basis of a histogram, and performing the tone conversion processing on the basis of the calculated tone conversion curve.
Furthermore, Japanese Unexamined Patent Application Publication No. 2003-69821 describes a technique of performing the tone conversion processing in consideration of a main subject by estimating a shooting situation. The publication further describes a technique of suppressing a side effect, such as noise, by imposing a predetermined restriction on the obtained tone conversion curve.
As the technique of reducing the noise, a variety of other proposals have been made. For example, Japanese Unexamined Patent Application Publication No. 2004-72422 describes a technique of performing high quality noise reduction processing by dynamically estimating the amount of the noise.
However, the conventional fixed tone conversion curve as described above involves a problem in that a suitable image signal cannot be obtained in a nonstandard situation, such as in backlight scene.
Further, in the techniques described in the foregoing Japanese Patent Nos. 3424060 and 3465226, the tone conversion curve is calculated for each image signal on the basis of the histogram. Thus, the increase of a noise component is not taken into consideration. Therefore, there is a problem in that optimal tone conversion processing is not always performed for image quality.
Furthermore, in, the technique described in Japanese Unexamined Patent Application Publication No. 2003-69821, the tone conversion processing is performed under the predetermined restriction. Thus, there is a problem in that, while a deterioration by noise does not occur in a normal image signal, a sufficient noise suppression effect cannot be obtained in an image signal of a wide dynamic range, for example. In addition, the technique described in the publication involves another problem in that constant performance of optimal tone conversion processing is not guaranteed, and thus that insufficient image quality may be obtained.
Further, in the technique described in Japanese Unexamined Patent Application Publication No. 2004-72422, the noise reduction process and the tone conversion process are independent of each other. Thus, there is a problem in that optimal mutual use of the processes is prevented.
The present invention has been made in view of the above circumferences. An object of the present invention is to provide an image processing apparatus and an image processing program capable of obtaining a high quality image signal by performing the optimal tone conversion in consideration of the change of the noise component.
Further, another object of the present invention is to provide an image processing apparatus and an image processing program capable of obtaining a high quality image signal by performing the optimal tone conversion in consideration of both the change of the noise component caused by the noise reduction process and the change of the noise component caused by the tone conversion process.